The long term objectives are to understand the molecular mechanics of cellular development in one particular case, in terms of how cells decide, and effect decisions, either to retain their original life cycle phenotype or to complete the transition to the succeeding life cycle phenotype. The water mold Blastocladiella produces motile, wall-less cells that do not grow (zoospores). These cells can either retain this phenotype or abruptly convert to immotile cells containing a cell will (cysts). The latter cells then normally complete life cycle prerequisites for entering the growth phase. The "choice" between remaining zoospores or becoming cysts can be made in the apparent absence of protein synthesis. Zoospores complete "starts" of the conversion to cysts in response to any one of certain extracellular "cues" (elicitors) but can then "stop", remaining motile and wall- less, in response to other cues (blockers). Stops are each reversible, by removing the given blocker, and starts are "remembered" during blocker- mediated stops. The elicitors and blockers are mostly compounds known to affect the functionings of low molecular weight "mediators" (cyclic nucleotides, calcium, phospholipid metabolites) in a wide variety of organisms. A simple procedure can be used to order the stops temporally. The organism itself releases a blocker, a newly discovered SH-containing cyclic ribotide that mediates a block easily distinguished from those mediated by cAMP, cGMP or any other known blocker. Research plans se presented to: a) define the structure, biosynthesis and mode(s) of function of the "natural" blocker; b) use doubly started zoospores to analyze how zoospores "process information" received from pairs of elicitors; and c) characterize cellular molecules involved in recognizing, transducing and using the information form different start (elicitor) and stop (blocker) signals. The latter plans include: a) uses of photoaffinity labels for cAMP and cGMP receptors; b) uses of cells accumulated at different stops to analyze stop-specific changes in protein phosphorylation-dephosphorylation and/or phospholipid metabolism; and c) isolations of elicitor or blocker-specific heritably conditional variants.